Differential signals are often used in high speed circuits including those associated with high speed interfaces. A differential signal is a signal that is constructed from a pair of signals: 1) a positive signal (the “+” signal); and, 2) a negative signal (the “−” signal). According to the operation of a differential signal, the − signal is designed to be the inverse of the + signal. FIGS. 1 and 2 demonstrate an example.
FIG. 1 shows a differential transmitter 101 (i.e., a device that transmits a differential signal) that is coupled to a differential receiver 102 (i.e., a device that receives a differential signal). A first signal line 103a is used to propagate the + signal (and may therefore be referred to as the + signal line 103a); and, a second signal line 103b is used to propagate the − signal (and may therefore be referred to as the − signal line 103b). Hence, the pair of signals are transported on a pair of signal lines 103a, 103b. The pair of signal lines 103a, 103b may be referred to collectively as a differential channel.
FIG. 2 demonstrates an example of the inverse relationship between a logical + signal and a logical − signal. Recalling that the − signal is designed to be the inverse of the + signal, note that the − signal 203b of FIG. 2 is the logical inverse of the + signal 203a of FIG. 2. That is, where the voltage level of the + signal 203a is a “1”, the voltage level of the − signal 203b is a “0”; and, likewise, where the voltage level of the + signal 203a is a “0”, the voltage level of the − signal 203b is a “1”.
FIG. 3 shows a depiction of a cross section 300 of a section of a planar board. Planar boards (which are also referred to as PC boards, PCBs, and the like) are used in a wealth of electronic products such as computing systems (e.g., laptops, personal computers, servers, workstations, etc.), networking systems (e.g., routers, bridges, switches, gateways, etc.), handheld devices (e.g., cellphones, personal digital assistants (PDAs), etc.) and test and/or measurement equipment. Planar boards are used to provide the signal lines that exist between a plurality of separate electronic components (e.g., discrete semiconductor chips, discrete capacitors, discrete resistors, etc.).
Typically, the separate electronic components are mounted to at least one side of the board (e.g., the “top side” and/or “bottom side” of the board). The inputs and/or outputs of these components (which are commonly referred to collectively as “I/Os”) are usually implemented as leads, balls, pins, etc. The I/Os are electrically coupled to one or more conducting regions that have been fabricated into the planar board. Thus (for example) if multiple semiconductor chips are mounted to the same planar board, the semiconductor chips can send a signal between each other via a conducting region that has been built into the board.
According to the planar board cross section 300 of FIG. 3, the board can be viewed as a multilayer structure having “signal” layers and “power/ground” layers. Note that the signal layers may also be referred to as signal planes 3011 through 3016; and, the power/ground layers may also be referred to as power/ground planes 3021 through 3023. Typically, the conducting regions used for propagating electrical signals reside along one or more of the signal planes 3011 through 3016. Each of the power/ground planes 3021 through 3023 are typically used for providing a reference voltage such as a ground reference or a Direct Current (DC) power supply voltage (e.g., 5v, 3.3v, 2.5v).
Thus, typically (although not a strict requirement), most of the conducting regions that reside along a signal plane resemble a collection of thin conducting strips (e.g., akin to wiring) whereas most of the conducting regions that reside within the power/ground planes resemble a wider planar region. Conducting strips are used to implement a signal line. In practice, a conducting strip often resembles a wire (e.g., having a substantially longer length than width) with a rectangular cross-section.
Usually, the planar board is constructed such that “neighboring” planes (e.g., signal plane 3011 and power/ground plane 3021) are separated by a dielectric material (e.g., located at region 303 of FIG. 3). The number of signal layers that reside between a pair of power/ground planes may vary from board to board (as well as vary within the confines of a single board as suggested by FIG. 3). The number of layers per board may also vary from board to board. It is also possible to design a planar board such that some power or ground references are supplied along a signal plane and/or some signals are transported along a power/ground plane.